1. Field of the Invention
This invention reveals an improved apparatus for natural dewatering of conditioned sludges in such a way as to decrease the time required for deliquification.
2. Description of the Prior Art
Natural drainage is the oldest method of reducing the water content of slurries and sludges. An excellent summary treatment of this subject is included in the Manual of Practice, No. 20 prepared by the Task Force on Sludge Dewatering, chaired by Charles T. Way, of the Water Pollution Control Federation in 1983 and is suggested as a reference for background.
Methods employed in the prior art include, among others, the use of open or covered lagoons for containment and evaporation, and sand drainage and drying beds, all of which are examples of the traditional and most simple methods. Properly operated, these methods are recognized as being capable of eventually yielding dryer residual solids than can any of the mechanical devices currently used for this purpose. However, problems inherent in the use of these methods, such as long residence time, large land area requirements, high periodic hand labor requirements, odor and insect control, and the like, have been resolved to various degrees of success by various mechanical, hydraulic and vacuum means, synthetic media, and other modifications and/or adaptations of the fundamental sand drainage technique. More recently, varied gradations of filtration media have been rigidified using synthetic resins for the purpose of providing fixed drainage surfaces that will support the weight of residual solids removal equipment without harming the underdrain water removal systems.
These devices have in themselves introduced new and significant problems in maintenance, operational continuity and reliability, and it is to this somewhat critical situation that the inventors herein have addressed their effort to develop a relatively low cost apparatus that is characterized by simplicity and reliability. Maintenance of the filtration and drainage surface is the key to operational reliability and objects of this invention provide the means to this end.
Returning to the prior art, the simplest method of consolidating sludge solids is to apply the slurry to a sand bed where the solids can be retained on the surface and the water filters away. The principle of supporting the surface filter sand with progressively coarser layers of gravel, from top to bottom, has been in practice of over 100 years. However, a sand bed is not desirable with the use of modern methods of moving residual sludge solids. That is, the spark of creativity since about 1950 has been the pursuit of methods that allow the use of the front-end loader or the like to remove the retained sludge solids without damaging the surface.
One method being marketed today is known as "vacuum assisted drying beds", which are implementations of Highstreet et al. (U.S. Pat. No. 4,431,549, issued Feb. 14, 1984); Riise (U.S. Pat. No. 4,481,114, issued Nov. 6, 1984); Stannard et al. (U.S. Pat. No. 4,340,478 issued July 20, 1982); Piper (U.S. Pat. No. 4,537,687, issued Aug. 27, 1985); and Piper et al. (U.S. Pat. No. 4,564,450, issued Jan. 14, 1986). An installation of such beds encountered, after an initial period of successful performance, plugging or blinding problems with the filtration drainage surfaces of the beds. After labor-intensive scrubbing, washing, backwashing and chemical cleaning and flushing, the beds were put back in service, only to experience the same problems after a short period of use. Subsequent intensive cleaning maintenance has resulted in diminished periods of satisfactory use, which are believed to be the result of the causes discussed below.
First, these beds are not functionally maintainable because the interstitial spaces between the media particles become fouled and washing them free of sludge matter either from the surface or by backwashing has proven only marginally successful. Normal backwashing opens a few paths through the media, but leaves the remained plugged, thereby allowing these few paths to be replugged when the media is put back in service.
Second, all of the patents referred to above specify the use of epoxy resin to rigidify the filtration matrix. Probably the earliest patent pertaining to a rigidifying drainage surface is U.K. Patent No. 1,403,186, Percharde, published Aug. 20, 1975, which specifies concrete as a binder, although epoxy is also mentioned. It is clear, however, that the matrix binder of choice in the prior art is epoxy resin, as evidenced by the above group of U.S. patents, which are all later than the British patent, indicating a turn away from any other binder.
Epoxy resin, however, when used as the matrix binder, will lead to the need for surface reapplications usually within the course of a year because of the destructive effects of ultraviolet exposure from sunlight. Regardless of how carefully surface reapplications are accomplished, progressive blinding of the interstitial surface spaces follows to ultimately defeat the original porous purpose intended.
Therefore, a feature of the present invention is to provide deliquification filtering apparatus for sludge that will not permanently foul or by simple means respond to cleaning to restore full surface function.
Another feature of the present invention is to provide deliquification filtering apparatus for sludge that includes a matrix media, the matrix binding material being a type that will not deteriorate as a result of exposure to ultraviolet radiation.